WO1989003983A1 - Process for determining and evaluating the combustion pressure of an internal combustion engine - Google Patents

Abstract

Conventional methods for determining the combustion pressure of an internal combustion engine are very costly and the sensors used must meet stringent requirements. The process described differs from this in that the combustion pressure is calculated by determining the compression pressure pk(alpha) and measuring the combustion chamber pressure p(alpha) by means of simple sensors. The combustion pressure p*(alpha) is obtained by subtracting the value of the compression pressure from the combustion chamber pressure. Preferably the variation in compression pressure is obtained by measuring the combustion chamber pressure during compression until the upper dead centre in the working phase is reached and storing the values in a memory. The values of the compression pressure after the upper dead centre is reached are obtained by reflection in an axis passing through the upper dead centre, which results in a symmetrical curve of compression pressure. The combustion pressure can be calculated very easily by this method.

Description

 Method for determining the combustion pressure of an internal combustion engine and for evaluating this pressure

State of the art

The invention relates to a method for determining and evaluating the combustion pressure according to the preamble of claim 1.

Methods for determining the combustion pressure are known, in which very robust and precise sensors are used. In particular, if the combustion pressure is to be used to control and / or monitor functions of the internal combustion engine, the requirements for the accuracy of the measured pressure values are very high. It is also essential that the sensitivity and accuracy of the measurement be constant over a wide temperature range got to. The known methods therefore have the disadvantage that very expensive sensors have to be used if the measured pressure values are to be used for the control and / or monitoring of the internal combustion engine.

Advantages of the invention

The method for determining the combustion pressure of an internal combustion engine with the features mentioned in claim 1 has the advantage that simple, relatively inaccurate pressure transducers can be used, the sensitivity of which changes during operation. This is achieved in that the combustion pressure is obtained by determining the compression pressure prevailing in the combustion chamber of a cylinder, that is to say the pressure which prevails in the combustion chamber without the influence of an ignition process. These pressure values are stored and subtracted from the combustion chamber pressure currently prevailing during operation of the internal combustion engine in order to generate the instantaneous combustion pressure value. All pressure values are determined depending on the crank angle ∝. The combustion pressure curve is thus calculated from the difference between the instantaneous combustion chamber pressure p (∝) and the compression pressure p _k (∝). Therefore, the combustion pressure is also referred to as the differential combustion pressure.

A method is particularly preferred in which the compression pressure is measured and stored as a function of the crank angle ∝ until a specific crank angle is reached. The ent standing pressure curve is mirrored on an axis running through the specific crank angle, the values stored before reaching the specific crank angle being called up as soon as this crank angle is exceeded. This compression pressure value, which runs along a symmetrical curve, is used to calculate the combustion pressure value. That is, the compression pressure values obtained in this calculation are subtracted from the current combustion pressure in order to determine the combustion pressure value.

This method can preferably be used to control the ignition angle in an internal combustion engine. For this purpose, the maximum of the combustion pressure value over the crank angle is determined. By shifting the ignition angle, this maximum value of the combustion pressure value is now shifted until it occurs at a desired crank angle. With this method, the so-called combustion position, i.e. regulate the energy conversion based on the crank position so that exhaust gas values and consumption of the internal combustion engine are as favorable as possible. The position of the combustion, the position of the combustion, is characterized by the point in time or the crankshaft position at which 50% of the energy of the cylinder charge is converted.

This method can preferably also be used for monitoring the internal combustion engine based on the combustion pressure. The maximum value of the combustion pressure is again determined and the cyclical fluctuations of this maximum value for monitoring the internal combustion engine used. It is also possible to carry out a smooth running control of the internal combustion engine on the basis of the cyclical pressure fluctuations obtained.

The method for determining the combustion pressure with the features listed in claim 11 has the advantage that the values required for determining the combustion pressure can be determined very easily. The measured combustion chamber pressure is integrated to calculate an integral value P * _{I of} the combustion pressure using two crank angle controlled integrators. A first integration range extends from a predefined crank angle over a specific crank angle range. A second integration area extends over an angular area of the same size lying in front of the determined crank angle. To calculate the integral value P * _I , the result of the integration lying before the determined angle is subtracted from the result of the integration lying after the angle.

In a particularly preferred method, the top dead center of a cylinder is selected as the specific crank angle during the working cycle.

drawing

The invention is explained in more detail below with reference to figures. Show it: Figure 1 is a diagram showing the position of the pressure maximum with respect to a defined combustion position;

FIG. 2 shows the flowchart of a first embodiment of a method for controlling the ignition angle in an internal combustion engine;

3 shows the flow chart of a second embodiment of the method according to FIG. 2;

FIG. 4 shows a diagram which shows the combustion chamber pressure over the crank angle ∝ (solid line), the curve of the combustion chamber pressure mirrored at an axis intersecting top dead center (dotted line) and the curve of the calculated combustion pressure (dashed line) at low load;

Figure 5 shows the course of the combustion chamber pressure and the calculated combustion pressure over the crank angle at high loads;

Figure 6 shows the course of the combustion chamber pressure and the calculated combustion pressure over the crank angle at low load;

FIG. 7 shows a flow chart from which it can be seen how the maximum value and the associated crank angle are determined from the combustion pressure;

8 shows the flowchart of a method for determining an integral value of the combustion pressure curve and FIG. 9 shows a flow chart of a method for determining the combustion pressure, in which an integral value is determined directly from the combustion chamber pressure.

Description of the embodiments

An essential parameter for the control and monitoring of an internal combustion engine is the combustion pressure prevailing in the combustion chamber.

In order to be able to determine the combustion chamber pressure so precisely that the measured pressure values can be used for the control and monitoring of an internal combustion engine, very precise pressure transducers which have a constant measuring sensitivity over a wide range must be used in known methods.

In the method according to the invention, the combustion chamber pressure p (∝) is determined using a simple pressure sensor. However, the pressure values determined are not used directly to determine the combustion pressure.

In addition to the combustion chamber pressure p (∝), the so-called compression pressure p _k (∝) is measured. Compression pressure is the pressure that arises in a combustion chamber due to the movement of the piston within the cylinder without ignition. The engine acts as a compressor, which is why the compression pressure is also referred to as compressor pressure. The compression pressure is measured at a certain crank angle, here up to the top dead center of the piston. These pressure values result in a curve over the crank angle ∝. This curve is reflected on a vertical axis that runs through top dead center.

This mirroring can be carried out graphically, for example, by plotting the course of the compression pressure P _k (∝) over the crank angle ∝ up to the top dead center and geometrically mirroring the curve shape obtained up to that point. It is also possible to enter the pressure values up to the top dead center in a memory. After reaching top dead center, the pressure values are called up that were measured and saved by the same crank angle amount before top dead center.

The compression pressure can also be detected continuously during the operation of the internal combustion engine. However, it must be ensured that no significant pressure changes are caused by an ignition process before top dead center. It is also possible to neglect such pressure changes. However, it is also conceivable to determine the compression pressure in a measurement process independently of the operation of the internal combustion engine, to store it in a memory and to mirror the curve shape obtained in the process.

From the measured combustion chamber pressure p (α) and the compression pressure p _k (α) of the combustion pressure P * (α) is calculated by the compression pressure p _k (α) is subtracted from the actual combustion chamber pressure p (α). The Instead of the actual combustion chamber pressure, the substitute quantity generated, namely the combustion pressure p * (∝), results from the following equation:

p * (∝) = p (∝) - p (∝). (1)

With the help of this method, a pressure curve is determined which is based solely on the combustion in the combustion chamber. It is particularly advantageous that the pressure can also be determined with the aid of relatively imprecise pressure sensors, the sensitivity of which changes during operation.

The compression pressure p _k (∝) prevailing in the combustion chamber can also be obtained in another way and used to calculate the equivalent pressure value. For this purpose, the polytropic compression or expansion is determined using the following equation:

In order to be able to determine the course of the equivalent pressure over the crank angle ∝, the compression pressure p _k (∝1) prevailing in the combustion chamber is determined at a specific crank angle ∝1 of, for example, 60 ° before the top dead center of the associated cylinder. This crank angle is chosen so that there is certainly no combustion in the combustion chamber. The combustion chamber volume V (∝1) is also determined at this crank angle. Otherwise, in equation (2) with p _k (∝) the compression pressure in a crank winch kel ∝, V (∝) denotes the combustion chamber volume at a crank angle ∝ and n the polytropic exponent, which in today's gasoline engines has a value of approximately 1.32.

Equation (2) can be simplified by replacing the expression in brackets with c (∝). So the following applies:

The values of c (∝) are shown in a table, i.e. stored in a memory. For each crank angle ∝, the associated values of c (∝) are called up to calculate the equivalent pressure p * (∝).

The equation (1) in conjunction with equations (2) and (3) gives the following function for the equivalent pressure p * (∝):

p * (∝) = p (∝) - c (∝) · p _k (∝1). (4)

One possibility for evaluating the pressure values obtained with this method is to be described using a method for controlling the ignition angle in an internal combustion engine.

In a known method of this type, the combustion position, that is to say the energy conversion based on the crank position, is regulated in such a way that exhaust gas values and consumption are as favorable as possible. The combustion situation is defined by the point in time or the crank position at which 50% of the Energy of the cylinder charge are implemented. The combustion position defined in this way is calculated from the combustion chamber pressure curve. However, this calculation is very complex. In conventional processes, the combustion position defined in this way is used to calculate the course of the combustion chamber pressure. In the case of known methods, in particular in the case of low load conditions, a clear association between the position of the combustion and the combustion chamber pressure maximum is practically not possible.

In a further known method of this type, the position of the maximum combustion chamber pressure, ie the crank position or the crank angle when the maximum combustion chamber pressure occurs, is therefore determined to determine the combustion position as a substitute variable. With a high load state of the internal combustion engine, a clear association between the position of the combustion and the pressure maximum can be found. However, this is not possible when the load is low, especially when idling. The reason is that the combustion chamber pressure change in the area immediately after top dead center is composed of a pressure drop due to the increasing combustion chamber volume and an increase in pressure which is based on the chemical energy released at that time. The energy is released by the combustion of the air-fuel mixture. The maximum of the combustion chamber pressure is reached when the pressure increase and pressure drop just cancel each other out.

By changing the ignition angle it can be achieved that the pressure maximum shifts and occurs at a different crank angle. Will the Ignition angle, i.e. the ignition point, changed, the combustion situation shifts. This also shifts the balance between the pressure rise and pressure drop mentioned. By changing this equilibrium position, the combustion chamber pressure maximum also changes.

Less chemical energy is released at low load. As a result, the pressure increase after reaching top dead center is less. The influence of the pressure drop based on the increasing combustion chamber volume is therefore relatively large. At low loads, the course of energy conversion changes depending on the combustion situation. This means that the position of the pressure maximum hardly changes with the position of the combustion even with a changing ignition angle.

If the above-described method for determining the combustion pressure is used to regulate the ignition angle in an internal combustion engine, the result is that the combustion position is easier to calculate and that even with a low load on the internal combustion engine, a clear association of the combustion position and crank angle with a changed ignition angle is possible . The pressure maxima used to determine the combustion position can be clearly assigned to different combustion positions in all areas, i.e. from high loads to idling, by changing the ignition angle. It is therefore easily possible, by changing the ignition angle, to regulate the position of the combustion in such a way that it can be assigned to a specific crank angle determined as optimal. This is achieved by first determining the maximum value of the calculated combustion pressure p * (∝) and the associated crank angle ∝ * _max . The ignition angle is now changed so that the maximum value of the combustion pressure occurs at a desired crank angle. In this case, ∝ * _max matches the desired crank angle.

It is particularly advantageous for this method for regulating the ignition angle if the values of the combustion chamber pressure determined during the compression phase, that is to say the compression pressure p _k (∝), are stored and the curve determined thereby is flipped or reached when a predetermined angle is reached, for example at top dead center. is mirrored so that there is a symmetrical curve for the compression pressure. This is then used to calculate the course of the combustion pressure p * (∝), which is used to determine the combustion position. A particularly simple control is achieved by this method.

To explain two types of embodiment of the method, reference is made here to FIGS. 1 to 6.

FIG. 1 shows a diagram in which the position of the maximum of the combustion chamber pressure over the position of the combustion, the combustion position, is recorded. Pressure maximum and combustion position are plotted over the crank angle ∝, whereby it is defined by definition that the crank angle ∝ assumes the value zero at top dead center during the combustion phase. Here too, the combustion situation is defined by the fact that 50% of the energy in the cylinder charge is converted.

Curve 1 is recorded when the internal combustion engine is under high load, curve 2 when the engine is under a low load. It can be seen that the relationship between the position of the pressure maximum and the position of the combustion in curve 1 is clear over a wide range. This is not the case with curve 2. The two curves 1 and 2 were recorded using a conventional measuring method.

In contrast, curves 1 * and 2 * were recorded using a method according to the invention. They represent the position of the maxima of the combustion pressure values obtained with the aid of the method according to the invention. It becomes clear that the association between the position of the pressure maximum and the position of the combustion is clear over a wide range, even with a low load on the internal combustion engine. The area of unambiguous assignment is also expanded for a high load state, as represented by curve 1 *. Curve 2 * represents a state of low load on the internal combustion engine. The pressure values for curves 1 * and 2 * are obtained using the procedure described above.

Instead of the current combustion chamber pressure p (∝), which changes depending on the crank position or the crank angle ∝, the combustion pressure p * (∝) is calculated. For this purpose, the course of the compression pressure p _k (∝) is determined, namely the pressure that in Dependence on the crank angle ∝ in pure compressor operation of the internal combustion engine, i.e. without the effect of the ignition. The compression pressure is subtracted from the actual combustion chamber pressure, so that the combustion pressure p * (∝) of the equation (1) given above results.

The position of the maximum value with respect to the crank angle is determined in any way from the course of the combustion pressure p * (∝). The associated crank angle is designated with ∝ * _max . The position of the maximum value of the combustion pressure obtained in this way is shown in curve 1 * for high loads and in curve 2 * for a low load state in FIG. 1. The position of the maximum value of the combustion pressure is assigned to the combustion position.

Figure 2 shows the associated flow chart, which is explained in the following:

For each crank angle ∝, the associated value of the function c (∝) given in equation (3) is retrieved from a table or from a memory S and multiplied by the value of the compression pressure pk (∝1) measured at a predetermined angle ∝ ₁ . The compression pressure pk (∝) thus obtained is subtracted from the actual combustion chamber pressure p (∝) to determine the combustion pressure p * (∝).

In a next step the maximum value of the

Combustion pressure P * _max determined. The associated crank angle Kurbel * _max at which the pressure maximum occurs is also determined. Another embodiment of the method is explained with reference to FIGS. 3 and 4:

To determine the compression pressure, which is referred to here as p _s (∝), the values of the combustion chamber pressure p (∝), which are measured before reaching a predetermined crank angle, for example top dead center, are stored with the associated crank angle. The resulting pressure curve is "flipped over" when top dead center is reached, that is, mirrored on an axis intersecting dead center and also stored in the memory. The course of the stored compression pressure P _s (∝) is shown in dotted lines in FIG. It can be seen from this illustration that the combustion chamber pressure p (∝) and the compression pressure p _s (∝) are identical until top dead center is reached.

In this embodiment of the method too, the compression pressure p _s (∝) is subtracted from the actual pressure prevailing in the combustion chamber to determine the combustion pressure p * (∝), so that the following equation (5) results:

p * (∝) = p (∝) - p _s (∝). (5)

The values of p _s (∝) are retrieved from the memory element S.

This can be seen from FIG. 3, which shows the flowchart of this method. According to this representation, the values of the combustion chamber pressure p (∝) become the Memory element S entered depending on the crank angle ∝. After reaching the predeterminable crank angle, here the top dead center, the values obtained on the basis of the curve mirroring are retrieved from the memory S as p _s (∝). In order to obtain the combustion pressure p * (∝), the difference between p (∝) and p _s (∝) is determined according to equation (5).

The course of the curve of the combustion pressure p * (∝) at low load, namely when idling, is shown in dashed lines in FIG. As in the embodiment of the method described above, the maximum value of the combustion pressure P * _max and the associated crank angle ∝ * _{max are} determined here.

FIG. 4 clearly shows the advantages of this method compared to conventional ones. The curve of the combustion chamber pressure p (∝) shown with a solid line reaches its maximum value at the top dead center and then only shows an irregularity in the falling part of the curve, that is to say the curve does not fall continuously. The combustion situation cannot be determined from this curve by means of a maximum value analysis.

If, on the other hand, one considers the curve of the combustion pressure p * (∝) shown in broken lines, a clear maximum can be ascertained without further notice. From the position of the maximum value, the combustion position can easily be deduced. The advantages of this method can be explained with reference to FIGS. 5 and 6. FIG. 5 shows the course of the combustion chamber pressure p (∝) and the calculated combustion pressure p * (∝) over the crank angle ∝ during a high load condition. In this representation, as in FIG. 6, a distinction is made between the pressure p ₁ (∝) and p ₂ (o) or p * ₁ (∝) and p * ₂ (∝). The deviating pressure curve between p ₁ and p ₂ or between p * ₁ and p * ₂ is based on a change in the ignition angle. At p ₂ or p * _{2 it is} shifted 20 KW late compared to p ₁ or p * ₁ . The maxima of the pressure curves are marked by vertical lines in the different curves. It becomes clear that when the ignition angle changes by 20 KW, the shift in the maxima in the course of the combustion chamber pressure p ₁ (∝) and p ₂ (∝) is smaller than in the course of the combustion pressure p * ₁ (∝) and p * ₂ (∝). This means that a control of the position of the combustion or the pressure maxima based on the inventive method with the aid of the ignition angle at a high load state is more effective than in conventional methods.

The advantages of the method according to the invention become even clearer when the internal combustion engine has low load conditions.

It can be seen from the curve profiles shown in FIG. 6 that the maxima of the combustion chamber pressure p ₁ (∝), p ₂ (∝) practically do not shift if the ignition angle during the measurement P ₂ (∝) compared to the measurement of p ₁ (∝ ) was postponed 20 KW late. In contrast, the maxima of the combustion pressure curves p * ₁ (∝) and p ₂ (∝) are clearly distinguishable. That is, in this method, the position of the combustion or the position can be changed by changing the ignition angle the pressure maxima are shifted until the position of the combustion or the pressure maximum is at the desired crank angle.

It should also be noted that when determining the compression pressure using the polytropic compression or expansion approach, the absolute pressure must be recorded. In addition, many pressure transducers are not suitable for static pressure measurement. Therefore, instead of the absolute pressure, the pressure prevailing in the intake manifold of the internal combustion engine during the intake phase is detected and, on the basis of this value, the pressure sensor is set to "0" during each intake process. The start of the suction phase can be recognized in FIG. 4 by the step-shaped pressure drop on the right in the diagram.

The method of mirroring the course of the compression pressure is unsuitable if there is a substantial pressure increase due to the combustion before reaching top dead center. In this case the compression pressure would have to be determined and stored in a separate process.

The pressure values obtained with the method for determining the combustion pressure of an internal combustion engine can also be evaluated for monitoring an internal combustion engine.

In known methods for monitoring an internal combustion engine, for example for determining cyclical combustion fluctuations and combustion misfires in an internal combustion engine, the indicated mean pressure is determined. This is very much on manoeuvrable, especially for processes in which engine monitoring takes place in real time.

In a further known method for monitoring an internal combustion engine, the pressure maximum is determined. In particular in the low load range of the internal combustion engine, it is no longer possible to make a statement about the cyclical combustion fluctuations, since, as explained above, the compression pressure dominates in this operating range and, in contrast, pressure increases due to the chemical conversion processes in the combustion chamber are only slight. The determination of misfires is therefore difficult. In order to differentiate between misfires and combustion, a high absolute accuracy of the pressure sensors is required, which is not the case with simple sensors.

If the above-described method for determining the combustion pressure is used to monitor an internal combustion engine, there is the advantage that pressure sensors with a relatively low accuracy can be used for determining combustion misfires, since only the sign, not the absolute level, of the pressure has to be evaluated .

If the values obtained with the described method for determining the combustion pressure are evaluated for monitoring an internal combustion engine, for example for determining the cyclical fluctuations in the work performed by the internal combustion engine, then the cyclical fluctuations in the maximum value of the combustion pressure p * (∝) are determined. However, it is also possible to integrate the combustion pressure p * (∝) over a crank angle range that extends from a crank angle before top dead center in the working phase to an angle after this top dead center. The cyclical fluctuations of this integral value P * _{I are also} determined here.

If combustion misfires are to be determined when monitoring an internal combustion engine, the sign of the integral value P * _I or that of the maximum value p * _{max of} the combustion pressure value is determined. To determine the maximum value p * _{max of} the combustion pressure value, the course of the combustion pressure is at least within a cycle comprising the working stroke, that is to say at least within one

Crank angle range of 180 ° determined. As soon as the integral value or the maximum value of the combustion pressure assumes a negative value, it can be concluded that there has been a misfire.

The combustion pressure can be determined particularly easily from a combustion pressure integral value P * _I with the aid of two integrations. The integration ranges extend over a certain crank angle range of, for example, 60 *. For the first integration, a predetermined crank angle is assumed, for example from top dead center ∝ _{0 of} the work cycle, and the second integral is integrated up to this angle. In this way, an integral value P * _I results according to the following equation (6):

This simplification is possible because the differential pressure curve can be expressed by the difference between two surfaces, with a reflection of the pressure curve at top dead center of the working cycle being possible as an alternative.

FIGS. 7 to 9 serve to illustrate the method for monitoring an internal combustion engine.

FIG. 7 again shows in a simplified manner how the maximum value P * _max and the associated crank angle ∝ * _{max are} obtained from the combustion pressure curve p * (∝).

The flow chart according to FIG. 8 illustrates a method for monitoring an internal combustion engine, the cyclical fluctuations of the work performed by the engine being determined on the basis of the cyclical fluctuations of the integral value P * _I (∝) or the maximum value p * _{max of} the combustion pressure curve. The cyclical fluctuations of the two pressure values can be detected, for example, by the standard deviation, the coefficient of variation or generally by the range of fluctuation. The cyclical fluctuations in the work performed can then be used for engine monitoring or for smooth running control. For this purpose, the angle ∝ _{max is} determined from the combustion pressure value p * (∝), at which the course of the combustion pressure within a

Working cycle comprehensive cycle, ie within a crank angle range of 180 º, a maximum. In addition to the current crank angle ∝ and the substitute pressure value p * (∝), two integrator crankshaft angles ∝ ₁ and ∝ _{2 are} entered into an integrator I. The integral value P * _{I is} calculated by this integration. With this integration, the integral value given in equation (7) results:

It is determined that the initial value ∝ _{1 of} the crank angle is less than or equal to the top dead center during the work cycle. For the second angle ∝ ₂ , an angle above top dead center is selected during the working phase, for example an angle of 60 °.

From the flow chart according to FIG. 9 it can be seen once again that the integral value of the combustion pressure curve P * I can be determined directly from the combustion chamber pressure p (∝) without the combustion pressure curve p * (∝) needing to be calculated in-between. With the help of the flow chart in FIG. 9, equation (6) can thus be implemented. For this purpose, the combustion chamber pressure p (∝) and the crank angles ∝ ₀ and ∝ ₀ + ∝ _{1 are} entered into a first integrator I ₁ . The current crank angle ∝ is also entered.

In addition to the combustion chamber pressure p (∝) and the current crank angle ∝, two angle values, namely ∝ ₀ - ∝ ₁ and ∝ _{0, are} entered into a second integrator I ₂ . The result of the second integrator is subtracted from that of the first, so that the value P * _I results as shown in equation (6).

The integral value obtained in this way can, as shown above, be used to control and monitor an internal combustion engine. The possibility of detecting cyclical fluctuations in the engine work output and combustion misfires also provides a method here with which protection for catalysts arranged in the exhaust gas stream of the internal combustion engine is created.

A diagnosis of the internal combustion engine is possible on the basis of the calculated values of the combustion pressure; in particular, torque components of the individual cylinders can be detected. The maximum values P * _{max of} the combustion pressure p * (∝) or the integral value P * _{I are used} as substitute values for the torque components. The differences in the cyclical fluctuations in the torque components of the individual cylinders can be determined correspondingly. From the different torque components or the various cyclical fluctuations in the torque parts of the individual cylinders can be derived from a cylinder-specific control, which influences, for example, the ignition, the mixture preparation, the injection quantity and the start of injection in diesel engines in such a way that the same torque components or the same cyclical fluctuations of the individual cylinders result. The equalization of the cylinders achieved in this way allows a particularly smooth, even engine running and a smooth running control to be achieved.

Claims

Expectations 1.Procedure for determining the combustion pressure of an internal combustion engine, the following steps are carried out: - The internal combustion engine pressure (p («χ)) which arises during normal operation of the internal combustion engine as a function of the crank angle (∝) and the compression pressure (p _k (∝)) which arises in pure compression mode in the combustion chamber are recorded; To determine the combustion pressure curve (p * (∝)) depending on the crank angle (∝), the difference between the combustion chamber pressure (p (∝)) and the compression pressure (p _k (∝)) is formed. 2. The method according to claim 1, characterized in that to determine the compression pressure (p _k (∝)) the combustion chamber pressure (p (∝)) of a piston until a certain crank angle is reached as a function of the crank angle (∝) is measured and stored, and that the stored curve profile is mirrored on an axis determined by a crank angle by calling up the values stored before reaching the specific crank angle symmetrically to the axis. 3. The method of claim 2, d a d u r c h g ek e n n z e i c h n e t that the top dead center of a piston is selected as a specific crank angle and / or that the axis is determined by the top dead center. 4. The method according to claim 1, characterized in that to determine the compression pressure (p _k (∝)) depending on the crank angle (∝) the polytropic compression or expansion is calculated, the values of the combustion chamber pressure (p _k (∝1)) and the combustion chamber volume (V (∝1)) can be used for a specific crank angle (∝1) at which no combustion takes place yet. 5. A method for controlling the ignition angle of an internal combustion engine by evaluating the combustion pressure obtained by a method according to one of claims 1 to 3, characterized in that the maximum value of the combustion pressure (p * (∝)) and the associated crank angle (∝ _max ) are determined and that the ignition angle is changed so that the maximum value of the combustion pressure occurs at a desired crank angle. 6. A method for monitoring an internal combustion engine by evaluating the combustion pressure obtained according to a method according to one of claims 1 to 3, the following steps being carried out: - The maximum value (p * _max ) of the combustion pressure and the crank angle (∝ _max ) given at this value are determined, - For engine monitoring, to determine misfires and / or for smooth running control of the internal combustion engine, the cyclical fluctuations of the maximum value are recorded. 7. A method for monitoring an internal combustion engine by evaluating the combustion pressure values obtained by a method according to one of claims 1 to 3, characterized in that an integral value (P * _I ) is determined by integrating the combustion pressure over a crank angle range, and that for engine monitoring, for detection misfires and / or to control the smooth running of the internal combustion engine, the cyclical fluctuations of the integral value are detected. 8. The method according to claim 7, characterized in that the crank angle range extends from a first angle (∝ ₁ ) before a certain crank angle, preferably top dead center during the working phase, to a second angle (∝ ₂ ) after the specific crank angle. 9. The method according to claim 6, characterized in that for determining combustion misfires, only the sign of the maximum value of the combustion pressure (p * _max ) is determined, negative values indicating a misfire. 10. The method according to claim 7 or 8, characterized in that the sign of the integral value (P * _I ) is determined to determine misfires, negative values indicating a misfire. 11. The method for determining the combustion pressure of an internal combustion engine includes the following steps: - The combustion chamber pressure (p (∝)) arising during normal operation of the internal combustion engine is recorded as a function of the crank angle (∝), - Then a combustion pressure integral value (P * _I ) is determined by integrating the combustion chamber pressure over two crank angle ranges and forming the difference between these integral values, the crank angle ranges being symmetrical to a predefinable crank angle (∝ ₀ ). 12. The method according to claim 11, characterized in that the top dead center of a cylinder (ZOT) is selected as a predeterminable crank angle (∝ ₀ ) during a working cycle. 13. The method according to claim 6 or 7, characterized in that for the realization a smooth running control, the cyclical fluctuations of the maximum value of the combustion pressure (p * _max ) or the integral value (P * _I ) are adjusted to each other with the aid of a cylinder-specific control. 14. A method for monitoring and / or regulating an internal combustion engine by evaluating the combustion pressure obtained by a method according to one of claims 1 to 3, the following steps being carried out: - The maximum value of the combustion pressure (p * (∝)) occurring within a cycle is determined and - The maximum values of the individual cylinders of the internal combustion engine are matched to one another by a cylinder-specific regulation. 15. A method for monitoring and / or regulating an internal combustion engine by evaluating the combustion pressure obtained according to a method according to one of claims 1 to 3, the following steps being carried out: - An integral value (P * _I ) is determined by integrating the combustion pressure (p * (∝)) over a crank angle range and - In order to achieve smooth running control, the integral value of the individual cylinders of the internal combustion engine is matched to one another by a cylinder-specific control. 16. A method for monitoring and / or regulating an internal combustion engine utilizing the combustion pressure obtained by a method according to one of claims 1 to 3, characterized by evaluating the difference between the combustion chamber pressure (p (∝)) and the compression pressure (p _k (∝)) . 17. Use of the combustion pressure integral value obtained by a method according to claim 11 or 12 for monitoring and / or regulating an internal combustion engine.